The nature of security, or access control, systems favors the centralization of all signal reception and authorization controls. However, in view of the variety of situations which are to be covered by a typical security system, control of all functions directly from a single computer control system becomes unwieldy and impractical for even the most modest systems. Previously, some of the signals could be consolidated in a simple remote module which is connected to the system functions to be controlled, such as door access and alarm signals, as well as being connected to the central system. However, while remote modules allow the hardware to be expanded, the increased data flow causes a further burden on the central processing computer system. Moreover, the entire system becomes vulnerable upon a power failure condition at the central computer. Typically, if the central computer power system goes down and the access to system records is maintained only at the central computer, a user is prevented from being granted entry at the remote unit because access to the database will be suspended during the power-down condition. In addition, breakdown in central/remote communications causes similar problems.
Alternatively, synchronized remote units allow entry to be granted if other remote units fail. However, the flexibility of a practical distributed security system is severely limited, or places a severe requirement on the communication system to be fast and accurate with the data transferred therein. Moreover, the redundancy of data stored among individual remote units for backup is relatively low, or if provided, causes significantly increased costs. Moreover, the format of the interconnection and synchronization of the various access control system elements to transfer data presents a problem to the structure of the communication channels, since the various system elements may operate somewhat independently. Furthermore, if the remote units are communicating to a central unit, the system operations and information exchange rates may be completely independent or at least different, requiring careful synchronization of the system elements or data handshaking protocol to achieve error-free data transfers.